Omeprazole is a gastric acid secretion inhibitor, useful as an anti-ulcer agent. U.S. Pat. No. 5,714,504 describes alkaline salts of (S)-omeprazole, such as sodium, magnesium, lithium, potassium, calcium or tetraalkylammonium salts. However, only the preparation of sodium and magnesium salts of (S)-omeprazole has been exemplified, besides (S)-omeprazole freebase in this patent. The potassium salt of (S)-omeprazole has been described as prepared in WO 98/54171 and WO 00/44744. The commercially available magnesium salt of (S)-omeprazole is used for treating and preventing peptic ulcers, gastroesophageal reflux disease (GERD or heartburn), erosive esophagitis, other conditions involving excessive stomach acid production, and for treating bacterial infections caused by helicobacter pylori.